The sky contains many active sources that emit X-rays, gamma rays, and neutrons such as our sun, radio galaxies, Seyfert galaxies, and quasars, as well as black holes, and clusters of galaxies. In addition to sources located in the heavens, many terrestrial applications are also associated with the penetrating characteristics of x-rays, gamma rays, and neutrons. Unfortunately hard X-rays, gamma rays, and neutrons cannot be imaged by conventional optics such as lenses or mirrors. As a result hard X-ray astronomy and other imaging applications were originally handicapped because of this lack of imaging capability. This led to the development of several innovative techniques including Fourier telescopes, one such telescope being the subject of U.S. Pat. No. 5,838,757. The theory and capability of Fourier telescopes is well understood. See the reference, “Imaging the Sun in Hard X-rays Using Fourier Telescopes” by J. W. Campbell, the inventor herein, found in NASA Technical Memorandum, NASA TM-108390 (January 1993). Fourier telescopes permit observations over a very broad band of energies that for photons range from the hard X-ray regime to very high energies up to and above several MeV. Depending upon the application, neutron sources across a wide band of high energies may also be imaged. For some applications, 1 eV neutrons may be sufficient while some applications may require imaging at energies up to and above 1 MeV to 100 MeV. In addition, complex sources emitting a mixture of these radiation types may be imaged simultaneously as well. These images may be integrated over all energy bands, or in one or more selected bands to aid in the understanding of the source characteristics. Thus a resulting integrated image may have a high spatial resolution as well as a high energy resolution.
In early approaches, multiple grid pairs were necessary in order to create rudimentary Fourier imaging systems. For example, 48 grids were used in a basic telescope design in Campbell, NASA TM-108390 at page 109. At least one set of grid pairs was required to provide multiple real components of a Fourier derived image, and another set was required to provide corresponding multiple imaginary components of the Fourier derived image. Image spatial resolution is limited by the widths of the grid slits (or slats). Requirements for better spatial resolution lead to exponential cost increases for grid fabrication and alignment.
It has long been recognized that the expense associated with the physical production of the numerous grid pairs required for its collimator was a primary constraint to achieving higher fidelity imaging. In addition, with imaging system aperture size often limited, improved sensitivity as opposed to higher fidelity and lower cost became an additional compromise. Thus, an innovative approach leading to a reduction in grid pairs and cost without sacrificing imaging sensitivity or fidelity was needed. This was accomplished in my U.S. Pat. No. 6,703,620 by creating Fourier derived images with only two grid pairs. The reduction to only two grid pairs needed for imaging was rendered possible by manipulating the grids through rotation and translation. Since a 24-grid pair Fourier imager can cost as much as ten times more than a two-grid imager to produce, the reduction in the number of grids is a significant reduction in cost. And, it was not believed that a one-grid pair Fourier imaging system was feasible because the first grid pair provides multiple real components necessary for a Fourier derived image, and the second grid pair provides corresponding multiple imaginary components for that Fourier derived image.
By this invention, a Fourier derived image can be generated in a system with only one grid pair. In U.S. Pat. No. 6,703,620 the possibility of utilizing only one grid pair was recognized and claimed. However, it was pointed out at the time that the single grid pair theory contemplated a collection of data at discrete, predetermined, points in the available spectrum based on estimating the imaginary component. Such guesswork leads to uncertainties in the accuracy of the final image and may actually result in a totally misleading image. For example, errors in a medical application such as the detection of breast cancer could go either way: (a) a tumor being undetected or (b) unnecessary surgery being indicated.